Snow

Snow is a form of precipitation within
the Earth's atmosphere in the form of crystalline
water ice, consisting of a multitude of snowflakes
that fall from clouds. Since snow is composed of
small ice particles, it is a granular material.
It has an open and therefore soft structure, unless
packed by external pressure. Snowflakes come in
a variety of sizes and shapes. Types which fall
in the form of a ball due to melting and refreezing,
rather than a flake, are known as graupel, ice pellets
or snow grains. Snowfall amount and its related
liquid equivalent precipitation amount are determined
using a variety of different rain gauges.

The process of precipitating snow is called snowfall.
Snowfall tends to form within regions of upward
motion of air around a type of low-pressure system
known as an extratropical cyclone. Snow can fall
poleward of these systems' associated warm fronts
and within their comma head precipitation patterns
(called such due to the comma-like shape of the
cloud and precipitation pattern around the poleward
and west sides of extratropical cyclones). Where
relatively warm water bodies are present, for example
due to water evaporation from lakes, lake-effect
snowfall becomes a concern downwind of the warm
lakes within the cold cyclonic flow around the backside
of extratropical cyclones. Lake-effect snowfall
can be locally heavy. Thundersnow is possible within
a cyclone's comma head and within lake effect precipitation
bands. In mountainous areas, heavy snow is possible
where upslope flow is maximized within windward
sides of the terrain at elevation, if the atmosphere
is cold enough.

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